Modern drilling operations have become marvels of technology and engineering science. The industry's efforts to maximize profitability range from initiatives that minimize “non-productive time” of drilling rigs and crews and maximize rates of penetration during the drilling process, to development of new methods for maximizing reservoir drainage and production rates. It is now commonplace for drilling crews to steer their drill strings along pre-planned or adaptively chosen borehole trajectories selected for optimum placement.
To the extent that crews can maximize the rate of penetration (without incurring additional non-productive time), they can complete their boreholes faster and, consequently, complete more boreholes within a given budget. One of the major factors for rate of penetration (though by no means the only factor) is weight-on-bit. Weight-on-bit is a measure of the amount of force that a drill string exerts on the bit face. It is a function of the configuration of the bottom hole assembly (including the size and number of heavily-weighted rigid drill collars), the weight and rigidity of the drill string itself, the hook load (the lifting force on the upper end of the drill string), the borehole size and trajectory, and a number of dynamic factors including frictional forces. As explained further below, these dynamic factors are affected by the drilling mode.
Rate of penetration is not a monotonic function of weight-on-bit. There is a “sweet spot” beyond which increasing the weight-on-bit actually reduces rate of penetration and eventually causes premature wear and damage to the bit. Similarly, weight-on-bit is not a monotonic function of hook load. As the hook load is reduced the drill string initially transfers its weight to the bottom hole assembly, thereby increasing the weight on bit. As the hook load is further reduced, however, the axial load along the drill string causes the drill string to bend, increasing the friction between the drill string and the wall. Further axial loads cause the drill string to buckle and eventually to reach a state referred to as “lock up”, where the frictional forces prevent any further progress along the borehole.
The complexity of this problem has led to the development of many methods and techniques for optimizing the rate of penetration. However, this complexity is magnified during the steering process. In particular, crews often have to transition between drilling modes as part of the steering process. For example, when maintaining the present course of the drill bit, crews employing bent-sub steering technology must operate in a “rotating mode” where the drill string rotates. To deviate from the present course, the crew transitions to a “sliding mode” where the rotation of the drill string is halted. (The drill bit continues to rotate due to the presence of a downhole motor.) Frequent transitions back and forth between the two modes are often required. Unfortunately, the different modes have different weight transfer characteristics due to different frictional forces and different buckling thresholds. Existing methods and techniques do not appear to adequately account for these differences, so crews have had to unduly limit their rate of penetration during the steering process.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.